Interpretive Summary: The boll weevil is an insect pest of cotton that originally invaded the U.S. from Mexico a little over a century ago. Tremendous effort and resources are being invested in eradicating the weevil from the U.S., and the threat of weevils flying into eradication zones from neighboring areas that are still infested is of great concern. Very little is known about how far a weevil is likely to migrate, and it is a difficult problem to investigate. To obtain information on weevil migration patterns, we used a molecular technique called ‘microsatellite analysis' to study variation in DNA from weevils collected across eight U.S. states and northeast Mexico. We found that migration is fairly frequent up to distances of 300 km. These estimates of the numbers of weevils that migrate are much lower than in a previous study that used a technique called mtDNA RFLP, but is only slightly higher than estimates from a study using a technique called RAPD analysis. Thus we judge the microsatellite and RAPD results to be the most reliable for this insect. Action agencies like USDA-APHIS can now use the microsatellite data collected in this study to determine the most likely geographic origin of a boll weevil captured unexpectedly in an eradication zone. This knowledge will help regulators decide the most appropriate response to prevent reinfestation.

Technical Abstract: The boll weevil (Anthonomus grandis Boheman) is an insect pest of cotton that underwent a well-documented range expansion across the southeastern U.S. from Mexico beginning about 110 years ago. Eleven microsatellite loci were surveyed to infer the magnitude and pattern of genetic differentiation among boll weevil populations from 18 locations across eight U.S. states and northeast Mexico. Estimates of genetic diversity (allelic diversity and heterozygosity) were greater in southern than northern populations, and were greater in the west than the east among northern populations. Boll weevil populations were genetically structured as a whole across the geographic range sampled, with a global FST of 0.241. South-central populations exhibit classic isolation by distance, but evidence suggests that populations within the eastern and western regions have not yet reached genetic equilibrium. Gene flow appears to be relatively high among populations within the eastern region. Estimates of gene flow indicate that migration between locations separated by <300 km is frequent. Spatial patterns of genetic variation among boll weevil populations revealed by microsatellites in this study are very similar to those derived from mtDNA PCR-RFLP and RAPD data reported earlier. However, estimates of migrant exchange between populations based on microsatellite data were generally intermediate to those derived from mtDNA PCR-RFLP and RAPD data, but much closer to the RAPD estimates than to the much higher mtDNA PCR-RFLP estimates. We conclude that microsatellite and RAPD markers provide a better picture of boll weevil gene flow than the mtDNA PCR-RFLP markers.